Conventionally, there has been a battery state detection unit that is connected to a battery via a bus bar and detects a voltage between terminals of the battery (JP-A-2006-032184). The battery state detection unit is configured such that a length of the bus bar is effectively short. With such a configuration, the battery state detection unit is less likely to have an influence of voltage drop caused by a resistance of the bus bar, and the voltage between the terminals of the battery can be further correctly determined.